The present invention relates to a developing apparatus for applying a developer onto a latent image formed on an image carrier to develop the latent image.
One-component developers or two-component developers are used in developing apparatuses of this type. A two-component developer includes a toner contributing to development and a carrier for properly charging this toner. However, in such a two-component developer, a mixing ratio of the toner to the carrier must be kept constant. In other words, the toner concentration must be kept constant. However, it is difficult to maintain a constant toner concentration. On the other hand, a one-component developer has an advantage in that the concentration control is not necessary, since only the toner for contributing to development is contained in the developer.
One-component developers are classified into magnetic and nonmagnetic developers. Magnetic developers contain magnetic materials in the nonmagnetic developer particles. When such a magnetic developer is used in a conventional apparatus, a magnet is arranged on the inside of a developer carrier for carrying the developer into the developing position and generating a magnetic field for supporting and carrying the developer. The following problems occur when the magnetic developer is used.
(1) The developer carrier becomes complicated, expensive and large since the magnet must be supported by the developer carrier.
(2) A magnetic developer containing magnetic particles is more expensive than a nonmagnetic developer.
(3) Since a magnetic developer contains magnetic particles which do not contribute to development, color reproducibility is not satisfactory. As a result, it is difficult to perform color development using a magnetic developer.
In order to solve the above problems, a developing apparatus using a nonmagnetic one-component developer is conveniently used. As a developing apparatus using a nonmagnetic one-component developer, U.S. Pat. No. 4,521,098 by Hosoya et al. is known. In the apparatus of Hosoya et al., a thin film layer of toner, as a nonmagnetic one-component developer which is formed on a developing roller, is pressed by only one blade, and the toner is triboelectrically charged by the blade. Thereafter, the toner is supplied to a photosensitive body on which a latent image is formed.
With the conventional developing apparatus as described above, since toner is triboelectrically charged only by one blade, all the toner particles cannot be sufficiently and reliably charged.
When the toner is insufficiently charged, fogging or scattering of toner particles occurs, and a clear image cannot be obtained.
When a toner image is successively transferred onto sheets, transferred images become blurred due to insufficient charging of toner.